This research is based on the hypothesis that cell-surface molecules play critical roles in the development and regulation of differentiating systems such as spermatogenesis. To explore this hypothesis, cell-surface differentiation antigens of spermatogenesis are being recognized and defined using monoclonal antibodies from hybrids derived by fusion of immunized lymphocytes with a myeloma cell line. Four antibodies have been found which bind determinants that are testis related and mouse-specific. The antigens are on all strains of mouse tested. By perioxidase-coupled antibody labeling of the antigen-bearing cells within a heterogeneous suspension of testis cells, the antigens have been shown to be present on the surface of the cells and to be on some, but not all of the cells in suspension.